Are you saying that the wiki is wrong?

It states in the "select" page that:

"If index has decimal places it gets rounded down for fractions less than or equal .5, otherwise it gets rounded up."

It states in the "random" page that:

"Random real (**floating point**) value from 0 (inclusive) to x (not inclusive)."

It also states in the notes section of the same page:

"x=round(random 5) will return 0,1,2,3,4 or 5. (non-uniform distribution, 0 and 5 are half as likely to be selected than any of the other numbers) x=floor(random 5) will return 0,1,2,3 or 4. (uniform distribution, all numbers have the same probability of being selected) x=ceil(random 5) will return 0,1,2,3,4 or 5. (0 is very unlikely, but possible, as ceil 0 is 0)"

You should round down. Making a test case with just 2 values isn't a fair test, as both are likely to to appear. Place in a 3rd element and then there is more probability that the middle element will appear.